With the development of Very Large Scale Integration (VLSI), lightly doped drain (LDD) and source/drain regions are employed in deep submicron systems. In order to make the source/drain expansion region shallower so as to control the short channel effect, a large amount of impurities are often used as dopant in an area near the channel of the source/drain PN junction to control the depth of the LDD region and the source/drain expansion region. However, implanting a large amount of impurities into the small region by the ion implantation method will cause defects, resulting in a leakage current in the PN junction and then damaging the semiconductor device.
In general, a secondary ion mass spectrometer (SIMS) method is used to perform a failure analysis for semiconductor devices. However, the SIMS method usually is time-consuming due to its low detection speed and therefore is not suitable for analyzing a small region. In addition, the SIMS method can only be used to detect defects in wafers other than semiconductor devices. Thus the detection result cannot faithfully represent defects in the final products.
Due to its high resolution, the TEM can be used to observe the patterns and dimensions of very thin films. Therefore, as the dimensions of semiconductor devices become smaller and smaller, especially when the device width is less than 0.13 μm, the TEM has become an important apparatus for observing and analyzing defects and structures in integrated circuits. FIG. 1A-1C are schematic diagrams of TEM samples fabricated by existing methods. As shown in FIG. 1A, a failure region 103 is positioned on the sample 100 by a method of electrical positioning. Two pits 101 and 102 having a larger area than the failure region 103 are dug out at both sides of the failure region 103 in the sample 100 using a focused ion beam (FIB) with a current of 7000 pA. As a result, the cross section of the failure region 103 can be observed during the subsequent process of milling the failure region 103, and the failure region 103 can be taken out from the sample 100 easily. The pits 101 and 102 are 15 μm×8 μm×6 μm (length×width×depth) (the dimension along the X-axis is defined as the length; the dimension along the Y-axis is defined as the width; and the dimension along the Z-axis is defined as the depth; the same below). The failure region 103 between the pits 101 and 102 is 3 μm-12 μm in length and 1 μm-3 μm in width. As shown in FIG. 1B, the current for FIB is adjusted to 300 pA and is used to mill the first surface 104 of the failure region 103, until the cross section of the failure region 103 for the semiconductor device is exposed. The milling depth is 4 μm. The second surface 105 of the observation region 103 is milled by FIB with a current of 300 pA, until the width of the failure region 103 is 80 nm-120 nm. As shown in FIG. 1C, the sample 100 is placed into a TEM observation chamber, and the failure region 103 is irradiated with an electron beam accelerated by a high voltage. The pattern of the failure region 103 for a semiconductor device is magnified and projected onto a screen for analysis.
The existing method for preparing TEM samples is disclosed by JP2004245841.
FIGS. 2A-2B are schematic diagrams of TEM samples prepared by existing methods. As shown in FIG. 2A, a cross section of the failure region for a semiconductor device is observed by a TEM with an amplifying multiple of 97000. The lightly doped drain 110 and source 112 are ion doped regions, and the silicon substrate 114 is a non-doped region. Since the ion doped regions are in the same thickness as the non-doped region, it is difficult to distinguish the lightly doped drain 110 and source 112 regions from the silicon substrate 114 and to clearly observe the pattern and defects in the lightly doped drain 110 and source 112 regions.
As shown in FIG. 2B, the pattern in the lightly doped drain and the source regions is observed by TEM with an amplifying multiple of 97000. The lightly doped drain 116 and drain 118 regions are ion doped regions, and the silicon substrate 120 is a non-doped region. Since the ion doped region are in the same thickness as the non-doped region, it is difficult to distinguish the lightly doped drain 116 and drain 118 regions from the silicon substrate 120 and to clearly observe the pattern and defects in the lightly doped drain 116 and source 118 regions.
Since the lightly doped drain, source/drain regions in the TEM sample prepared by FIB according to existing methods are in the same thickness as the silicon substrate, it is difficult to distinguish the lightly doped drain, source/drain regions from the silicon substrate and to clearly observe the pattern and defects in the lightly doped drain, source/drain regions.